There are various products which require thermal insulation during shipment, one of the more common of these being frozen food stuffs. Even though the quality of insulating material and techniques have improved over the years, the thermal insulation provided by present day commercial shipping containers is not able to maintain the contained product within the proper temperature range over longer periods of time, without using refrigerating techniques or some other means in addition to providing insulation.
It has long been known that excellent insulating capability can be obtained by providing a vacuum between two members, a common device utilizing this principle being the vacuum flask. Such a flask is made up of inner and outer walls which are spaced from one another, with a vacuum being provided in the space between the two walls. Primarily for structural reasons, the two walls are formed as concentric cylindrical sidewall sections, with the ends of the cylinders being closed by concentric hemispherical sections. An opening is provided through one of the end hemispherical sections.
However, the walls of even a relatively small vacuum flask are subjected to rather substantial forces. With the atmospheric pressure being approximately fifteen pounds per square inch (PSI) at sea level, the outside wall of a three inch diameter by twelve inch long standard vacuum bottle is subjected to a total lateral force of as much as approximately 540 pounds. The internal wall of the flask does not require as heavy a wall, since the internal forces are directed radially outwardly, so that the material forming the inner wall is in tension, with there being no buckling tendency. However, the outer wall experiences what can be described as a crushing force, and the outer wall must be made structurally stronger to withstand the forces which would tend to buckle the outer wall. Further, the structural problems become more difficult to solve as the size of the container becomes larger. The structural problems and other related problems in designing a vacuum insulated container in other configurations are often even more substantial.
Another factor is that while cylindrical containers may be reasonably practical for shipment of fluids, the cylindrical containing area is less practical for other types of cargo. Further, when a number of such cylindrical containers are stacked in a cargo area, there is much wasted space between the containers.
Also, there are a number of other design challenges in making an economically feasible shipping container, such as structural strength and durability, economy in manufacture, and other factors. Because of the structural problems and-other problems of providing commercially practical vacuum insulating shipping containers, in many instances the thought of using the evacuated area as insulation is abandoned, and thick high quality insulation is used. Also,.for practical reasons and also for utilizing the cargo space to full advantage, shipping containers are commonly made rectangularly shaped. The end result is (as indicated above) that to maintain quite low temperatures (or more broadly to maintain substantial temperature differentials between the contained cargo and the ambient atmosphere) for long periods of time, even the use of quite thick high quality insulation of itself has not been adequate, and refrigeration or other techniques must be utilized.